Ive got 2 thermostats hooked up to 2 heat pump style AC units at my house. Over the weekend I impressed myself and installed a Lyric thermostat to replace a broken thermostat (on the inside of my house) to one of the AC units. It went so easily, I want to replace the other, probably with a Lyric T5.

In any case, the one I did, the wiring was a pretty simple, one to one relationship, where I could match most things, but it did take a few trips to the air handler to find my C wire. The second thermostat doesnt seem to be wired as simply as you can tell by the photos below hopefully.

I think my C wire is black and hiding behind the thermostats housing, any confirmation on that would be great:)

I can see that my R terminal and my O terminal both have 2 wires in the connection to the board. I also notice in the pic of the current thermostat, that the red wire is split and going to an RH and RC connector. I didnt have that on the first thermostat, so Im wondering what to make of all this?

Some other things of note that I dont understand in the pic of the thermostat as well:

  • I dont have any kind of gas heat for anything, not water, not anything but it looks like a yellow and blue wire go into that contact on the thermostat.

  • The white wire runs to a contact thats labeled electric and while the unit is run by electricity, Im not sure what that one is for.

  • Lastly, that green wire runs to nothing.

I would appreciate any help anyone can give me as to why some of these contacts have dual wires running into them as well as what goes on with the gas/electric portion as well as the green wire, of that thermostat.

Thanks very much in advance!



1 Answer 1


Rc and Rh – split connections for 24vac if the system has separate cooling and heating control transformers, respectively. Most systems have a single control transformer on the heating side only and in that case Rc and Rh usually are shunted together at the thermostat as shown.

Elec/Gas – this is a label for the switch below the labeling, not the wire terminals above. If you do not have a gas furnace, this switch should be set to “elec” as shown.

Blu + Ylw wires – these look like they are connected to the thermostat terminal marked “Y”. Look above the lower set of terminals under the wires. “Y” energizes the compressor for cooling mode. “W” normally energizes the furnace, but since you have a heat pump these wires are shunted together (compressor needs to be on in either cooling or heating mode).

Wht + Grn wires – one of these probably runs to the “O” terminal and the other runs to the “G” terminal. I cannot tell which is which from the thermostat photo. Based on the other photos, green connects to “G,” which controls the indoor blower and is typical wiring. “O” controls the reversing valve. When “O” is energized, the outdoor unit is in cooling mode. When “O” is not energized, the unit is in heating mode. In rare cases the terminal is marked "B" instead of "O," which has complimentary functionality relative to "O."

C wire – Based on the above photos, the black wire is in the bundle of thermostat wires somewhere, probably hidden behind the thermostat as you suggest.

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